WASHINGTON — The fear of reprisals against his parents might have caused Ukrainian sailor Miroslav Medvid to decide to return to the Soviet Union, Secretary of State George Shultz said Sunday.

As Medvid's ship steamed toward home, Shultz defended the administration's handling of the ship-jumping incident and said that everyone involved, including the Justice Department, had concluded that Medvid had changed his mind about defecting.

The situation created problems for U.S.-Soviet relations shortly before the summit between President Reagan and Soviet leader Mikhail Gorbachev in Geneva Nov. 19-20. It also drew sharp protests from the Republican Party's right wing, led by Sen. Jesse Helms, R-N.C.

The 120,000-ton grain-carrying freighter Marshal Konev was allowed to leave Mississippi River waters in Louisiana on Saturday.

It sailed into international waters early Sunday despite a subpoena from the Senate Agriculture Committee. Protest boats were watched by the Coast Guard to prevent any trouble.

''It would look as though he decided somehow that he wanted to come to the United States and after he was subjected to whatever he was subjected to aboard that ship, he changed his mind,'' Shultz said on CBS' Face the Nation. The secretary called attention to the report of a U.S. Air Force psychiatrist who interviewed Medvid after the sailor first was returned to the ship by the Border Patrol and then removed by U.S. authorities for further questioning about his intentions.

Shultz noted that Medvid ''repeatedly referred to Mama and Papa, and he talked about life. I have no basis for being explicit about what threats may have been suggested to him. But it is clear that he had his mother and father on his mind, and, all things considered, decided what he wanted to do.''

Shultz conceded, as he had done previously, that it was a mistake to return Medvid to the ship against his will after he had twice jumped into the Mississippi.

But Shultz asserted that everything possible subseqently was done to correct that mistake.

He noted that Medvid had been taken off the vessel, was allowed to rest, was given physical and psychological examinations and was questioned anew ''in a non-threatening environment'' about whether he wished to remain in this country.

The 25-year-old Medvid was returned to his vessel by Border Patrol agents after jumping ship at Belle Chasse, La., on Oct. 24.

On Oct. 28, Medvid was removed from the ship for lengthy interviews with State Department officials and eventually signed a statement declining political asylum.

Helms, chairman of the Agriculture Committee, issued a last-minute subpoena for Medvid. But Shultz said the final decision on allowing the Marshal Konev to depart was up to the Justice Department.

Shultz said, ''It was the consensus of all the people working on this -- representatives of the Department of Transportation, Justice, State and so on -- that there was no legal basis to hold the ship.''

Shultz also was questioned about Vitaly Yurchenko, the KGB official who defected to the United States last August only to turn up last week at the Soviet Embassy in Washington, claiming he had been kidnapped, drugged and brought to this country against his will.

''He is a high person in the Soviet KGB,'' Shultz said in response to questions about whether Yurchenko might have been part of a plot to embarrass the United States on the eve of the summit. ''My opinion is that he defected and for some reason changed his mind.''

Shultz refused to discuss what the United States had learned from Yurchenko during the three months he was in U.S. custody. ''But he was not given any U.S. secrets at all,'' Shultz said.